Hydroprocessing such as hydrodesulfurization, hydrodenitrogenation, hydrodeoxygenation, hydrodemetallation, hydrodearomatization, dewaxing, hydroisomerization, and hydrocracking, is important commercially to upgrade crude hydrocarbon feedstocks. For example, hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), are used to remove sulfur and nitrogen, respectively, and produce clean fuels.
Conventional hydroprocessing processes use trickle bed reactors in which hydrogen is transferred from a vapor phase through a liquid phase hydrocarbon feed to react with the feed at the surface of a solid catalyst. Thus, three phases (gas, liquid and solid) are present. Trickle bed reactors are expensive to operate and require large quantities of hydrogen, much of which must be recycled through expensive hydrogen compressors. Heat removal from the highly exothermic hydroprocessing processes is inefficient. Significant coke forms on the surfaces of catalysts in trickle bed reactors, causing catalyst deactivation.
U.S. Pat. No. 6,123,835 discloses a two-phase hydroprocessing system which eliminates the need to circulate hydrogen through the catalyst. In the two-phase hydroprocessing system, a solvent or a recycled portion of hydroprocessed liquid effluent acts as diluent and is mixed with a hydrocarbon feed. Hydrogen is dissolved in the feed/diluent mixture to provide hydrogen in the liquid phase. All of the hydrogen required in the hydroprocessing reaction is available in solution.
Two-phase hydroprocessing systems contain a single liquid recycle stream to increase dissolved hydrogen availability throughout a reactor. The recycle stream eliminates hydrogen gas recirculation through the catalyst and provides a heat sink for a uniform temperature distribution. However recycle has disadvantages. Recycle introduces back-mixing to the system, which reduces conversion, for example, sulfur removal efficiency. Back-mixing reduces catalyst efficiency because reaction products, such as hydrogen sulfide and ammonia, which are present in the recycle stream take up the catalyst active sites. This causes difficulty in competing with conventional trickle bed reactors, which do not have liquid recycle, in kinetically limited regions, i.e., reducing sulfur below 10 ppm for ULSD. By “kinetically limited region”, it is meant herein where organic sulfur concentration is very low (such as around 10-50 ppm). The reaction rate of organic sulfur conversion is reduced, kinetically limited, at such low sulfur concentrations in the presence of recycle, which includes reaction products.
It would be desirable to have, and the present invention aims to provide, a two-phase hydroprocessing systems which reduces or eliminates the need for a recycle stream and allows increased sulfur and nitrogen conversions.
U.S. Pat. No. 6,428,686 claims a hydroprocessing process comprising combining a liquid feed with reactor effluent and flashing with hydrogen, then separating any gas from the liquid upstream of the reactor and then contacting the feed/effluent/hydrogen mixture with a catalyst in the reactor, removing the contacted liquid from the reactor at an intermediate position, combining the removed liquid with hydrogen gas to resaturate with hydrogen, separating the gas from the liquid and reintroducing the removed liquid back into the reactor at the point the removed liquid was withdrawn.
U.S. Pat. No. 6,881,326 claims a hydroprocessing process comprising combining a liquid feed with reactor effluent and hydrogen so that the hydrogen is dissolved to form a substantially hydrogen-gas-free liquid feed stream and then contacting the liquid feed stream with a catalyst in the reactor with substantially no excess hydrogen gas present removing the contacted liquid from the reactor at an intermediate position combining the removed liquid with hydrogen so that hydrogen is dissolved within the removed liquid and reintroducing the removed liquid back into the reactor.
U.S. Pat. No. 7,569,136 discloses a continuous liquid phase hydroprocessing process. In one embodiment, a down flow two-reactor system is described wherein feed, recycled reacted product and hydrogen are combined in a first mixer and the first mixture flows to a first reactor; the product from the first reactor is combined with hydrogen in a second mixer and the second mixture flows to a second reactor. In another embodiment a down flow multi-bed reactor system is described wherein feed, recycled reacted product and hydrogen are combined in a first mixer and the first mixture flows into the reactor and through a first catalyst bed; the product from the first reactor is combined with hydrogen in a second mixer and the second mixture flows to a second catalyst bed.
Although processes are known for liquid phase hydroprocessing, there remains a need for improvements, for example, higher conversions with less back mixing. The present invention meets this need.